Our copending application U.S. Ser. No. 08/074,826, filed Jun. 11, 1993, describes a method of developing an imagewise exposed silver halide color material to provide sensitometric results of reduced variability which comprises carrying out color development in the presence of one or a combination of black-and-white silver halide developing agents (termed herein electron transfer agents or ETA's) incorporated in said silver halide color material in an inactive form from which the active form is released during processing. The presence of the agent causes the effect that development is accelerated under low activity development conditions which would normally decrease the rate of development while development is decelerated under high activity development conditions which would normally increase the rate of development. Thus the agent has the effect of reducing the variability in the density versus LogE curve (the characteristic curve) caused by changes in development process variables such as time, temperature, color developing agent concentration and bromide ion concentration. Model examples having the black-and-white silver halide developing agent or ETA present in the developer solution are described in the copending application.
The prior application mentions that, when the black-and-white silver halide developing agent is incorporated in the photographic material, it is preferably in a form which is inactive until processing takes place. For example it may be inactivated by a blocking group which is hydrolyzed off when the material is immersed in the developing solution (which is alkaline).
The specific examples of this application demonstrate the effect of the invention using model experiments with developer solutions containing the effective black-and-white developing agents.
Problem to be Solved by the Invention
The problem with known hydrolyzable blocked pyrazolidone developing agents is that they only unblock at the required pH if the compound is rather unstable. If the compounds are stable enough not to break down in the material, they do not unblock fast enough (or at all) to be useful. A problem to be solved is to provide a photographic material containing an ETA which is stable during keeping but which is capable of unblocking during development in order to help minimize the effects of development process variables on image density.